Instrument landing system



W. L. BARROW INSTRUMENT LANDING SYSTEM SS JWN NO NNW May 15, 1951 Filed Jan. 25, 1947 .N mvg W/L Mn? L. EHR/@ow BL/W /QTTOR/VE'Y Filed Jan. res,- 1947 s snets-sheet, :s

May l5, 1951 w, BARRQW 2,552,511

INSTRUMENT LANDING SYSTEMv W/L MEA l. EAI/wow o; BY SQ @ymf TTOR/VE Y Patented May 15, 1951 INSTRUMENT LANDING SYSTEM Wilmer L. Barrow, Manhasset, N. Y., assigner to The Sperry Corporation, a corporation of Dela- Ware original application october 23,1941, serial No. 416,160. Divided and this application January 25, 1947, Serial N0. 724,389

6 Claims.

This invention relates, generally, to the blind or instrument landing of aircraft by radio means employing an overlapping beam type of instrument landing system in which the signal transmitted from the ground or other landing area and received at the aircraft by novel receiver means thereon is designed to provide in the craft suitable control voltages or currents such as an audio frequency voltage or current of reversing phase character. Such a signal is suitable for use in a servo-controlling means which controls the landing of the aircraft and in actuating indicating instruments useful as aids in landing aircraft.

In the overlapping-beam instrument landing systems of the prior art, two or more beams are transmitted to provide an equi-signal course along which the plane should fly in order, safely and properly, to carry out an instrument landing. In these prior art systems, it has been customary to modulate each of the two or more beams at distinct and different audio frequencies; for example, 90 and 150 cycles, etc., a second. Further characteristics of these prior-art systems include separation or filter circuits in the receiver that separate into distinct circuits the received signal from each of the two or more beams, and indicating devices that operate by virtue of the difference of intensity of the signals thus separated. It is also customary, in most of the prior art systems, alternately to transmit on one beam and then the other, in order to avoid effects of interference in space by the two beams of the same carrier frequency. This commutation of the carrier necessitates the use of more or less complicated equipment and in addition generally lowers the operating efficiency of the systems employing the same. Further, these prior art systems generally were not readily adaptablefforautomatic control of the aircraft.

The present application is a division of Patent No. 2,414,791 for Instrument Landing System, filed October 23, 1941 in the name of Wilmer L. Barrow.

An object of the invention is to provide a novel receiver for aircraft adapted for receiving radio beams and utilizing the same for establishing the degree of deviation of the craft from its true course in any desired plane or planes, said receiver producing a variable magnitude, reversible phase signal suitable for control purposes.

Another objectof the invention is to provide means for utilizing the output of said receiver for controlling servo means eifecting automatic control of the craft.` Y l Another object of the present invention is to provide in a beam type of instrument landing system of the above character wherein the ultra high frequency transmitter means employs an audio frequency or servo-signal modulation superimposed on a radio frequency modulation -to make possible a separate reception of the indicating or servo-signal from the reference signal, which has the same audio frequency and must be transmitted independently on the craft to provide the necessary phase reference required by said novel indicating or servo-signal electrically controlled equipment, said reference signal modulation being superimposed on a different radio frequency modulation from the servosignal, thus providing distinctchannels which may be filtered in the receiver means and independently demodulated.

Another object of the invention is to provide a novel high frequency receiver adapted to receive carrier signals containing reference and signal side bands, together with means for comparing said reference signal and side bands as to phase, and the servo mechanism controlled from said phase comparing means.

The invention also relates to the novel features or principles of the instrumentalities described herein, whether or not such are used for the stated objects, or in the stated fields or combinations.

Other objects and advantages will become apparent from the specification, taken in connection with the accompanying drawings wherein the invention is embodied in concrete form.

In the drawings, y

Fig. 1 is a block diagram of a transmitter ofA this invention suitable for producing the desired beams in either azimuth or elevation.

Fig. 2 is a diagram of a receiver suitable for changing right-left and/ or up-down information, as received from two transmitting systems such as the one shown in Fig. 1, into servo signals for operation of the control surfaces of the aircraft by means of a conventional servo-system and/or for operation of indicating instruments.

Fig. 3 shows the plan view of the three beams required for control in any one coordinate.

Fig. 4 shows the character of the modulation applied to these three beams.

Fig. 5 shows the variation of the final direct current output of the receiver used in controlling the servo-system as a function of the angular deviation of the aircraft from the fixed ight path.

Fig. 6 shows a cross section of the five beamsV needed for control in both coordinates.

Figs, 7, 8 and 9 show vector relations from which Fig. 6 is derived.

Fig. 10 illustrates an electric servo device adapted for use in connection with the structure of Fig. 2.

Referring now to Fig. 1, a block or functional diagram is illustrated of the instrument landing system as pertains to one coordinate or plane. The transmitting source I is a frequency stabilized radio-frequency oscillator of. conventionaltype, whose frequency, after multiplicationv by the ultra high frequency multiplier amplifiers 2, 3 or 4, becomes a frequency which. is of the order of 3 109 cycles per second and which may be designated as w and is useful in this invention because of the well known properties of such ultra high frequency. The output of a modulation oscillator 5, preferably of radio frequency is modulated by the output ofI a lower frequency oscillator 8 and the resultant modulation prod-- ucts are used to modulateV the carrier frequency of device 2. Thus, for example, the frequency w emitted by multiplier amplifiers 2 and 3 may be modulated by a frequency 181 (of the order of, say 300 kilocycles), i another frequency a (of the order of, say 60y cycles). The modulation a applied from oscillator 8 has =0, where qa is the phase; this signal, combined with c1 in modulator 6 is referred to as miao. The useful side bands supplied b-'y multiplier 2 are then ciclico. The modulation a applied from oscillator S. has =180; this signal, combined with ci in modulator 'I is referred to as inician. The useful side bands emitted by multiplier 3 are then wiltaiao.

The frequency w emitted by multiplier amplifier ll is modulated. by av frequency z (of the order of, say 109 kilocycles) i the a frequency. The modulation a. applied from oscillator I5 has =R which may have any fixed phase relation to the outputs of oscillators 8 and 9; this signal, combined with 462. in modulator oscillator M is referred to as zi-aa. The doubly modulated carrier emitted by multiplier 4 is then wiziaa. The three signal channels, wiif-ao, wimmo, and wim-aa are fed through wave guides II, .IIIy and I5, to directive electromagnetic radiators I2, I3 and IT, respectively. These radiators are shown as horns but may be any of the well known types of directive radiators. The radiators I2 and I3 are so orientated relative to each other that the `characteristic overlapping-beam radiation pattern consisting of beams I8 and I9 (see Fig. '3) is produced, whereas radiator II produces a wide beam 28 encompassing both beams I8 and i9. These beams are transmitted continuously, and the lower frequency a of modulation of the three beams is the same. The phase of the three modulations is such that when the envelope of one beam has its maximum, that of the second pre yerably has its minimum, while the envelope of the third or reference beam, whose phase is the reference, may be similar to one of the above o1` at an intermediate value.

Two distinct pairs of overlapping beams I8, I9 and I8', I9 (see Fig. 6) are employed for aircraft landing guidance as follows: one pair I8', I9 displaced angularly from each other in a vvertical plane intersect along an inclined line 'producing the desired flight path to give altitude or glide path information to the craft; and the other pair I8, I9 displaced angularly from each other in a transverse plane, also intersect along 4 said inclined line to give lateral or runway localizer information to the craft. These two sets of beams together provide continuous indication of the relative position of the plane with respect to the reference landing path in space that intersects the runway at its leading cir-approach edge.

Thus, twol of the systems shown inv Fig. 1 would be necessary for the complete system described above. A cross-section of the ve beams necessary being shown in Fig. 6. The beams have the following frequencies:

Right-left: mimi-a0 Beam I8 vi-,Briatico Beam I9 Up-down: wip/xiao Beam I9 wisiiso Beam I8' Reference.; wit-ian Beam 20 Obviously, the same 0.a signal can be used for both coordinates thus eliminating one reference beam channel.

Suppose we consider again the right-left signal generation system as shown in Fig. 1. If, for example, the radiators I2 and I3 are to supply this right-left signal, then Fig. 3y shows the horizontal patterns of the radiation fields, curve I8- indicating the pattern from radiator i2, curve I9, that from radiator I3, and curve 2D, that from radiator I'I".

According to the above description, antennas l2 and I3 transmit overlapping beams in which the indicating or servo-signal modulation a0,- am is superimposed on the relatively high separation frequency modulation of frequency [31 from modulator 5. The purpose of the modulation oscillator 5 is to make possible a separate reception of the servo-signal, i. e., the vector sum of ac and also which may be designated as ap, of the overlapping beams la and I9 from the reference signal an of beam 2E), which has the same frequency as the servo-signal and which must be transmitted to the craft also. In the functional diagram shown in Fig. 1, the separate 'transmission of the servo and reference signals is provided by means of the two distinct double modulations of frequencies [31 and z. Appropriate equipment, to be described, in the receiver separates [il and its servo signal modulations from z and its referencesignal modulations. f

Fig. 4 illustrates the relation of a modulating voltages of the outputs of antennas I2, I3 and I1, respectively; the upper diagram showing the phase of the a modulation on beam I8, the middle diagram, the a modulation of beam I9; this being of opposite phase; and the lowest diagram showing the phase of the reference modulation a on beam 20. y

The operation of servo-signal electrically controlled equipment of this invention requires the use of the aforesaid servo voltage and the steady reference voltage of identical frequency. The reference voltage after reception by the aircraft is to be continuously supplied to servo apparatus of a balanced rectifier or similar type in order that the latter provide a direct current output whose amplitude and polarity vary with the magnitude and phase, respectively, of the resultant servo signal ai. The pattern 20, representing the reference beam is preferably made broad to supply said reference voltage at all positions of the craft in space at which the servo-signal may be received. Frequencies i and [i2 are chosen to make modulation simple and to make easy the separation of servo and reference signals in the receiver, as determined by the details of the equipment .employed and the state of theart.

Fig. 2 illustrates practical receiver equipment for this blind landing system. The energy received, contains the following'frequencies, for a ve beam system, i. e., @,wiiiar,

wiziamwikiaf where we denoted an, also by ap, and is picked up by antenna 2|, then amplified and demodulated by ultra high frequency receiver 22. The output of receiver 22 consists of iiap, ziaa, and

[S31-ap resulting from the above-described mod-V ulation processes. The output of the receiver 22 is separated into three different circuits by means of the separation filters 23, 24 and 25. Filter 23 selects iiap and rejects all other signals. The second separation filter 24 passes ziaa. The third filter 25 passes signals of siap. The three filters are followed by demodulators 26, 21 and 28 whose output will contain currents of frequency a only.

The outputs of the demodulators 26, 21 and 23 are amplified by means of power amplifiers 29, 30 and 3|, respectively. A cathode ray indicator 32 of the type disclosed in Patent No. 2,262,033 of F. Moseley for Aircraft Flight Indicator and Control System 'Iherefor and Patent No. 2,384,484 of E. Norden, F. Gemmill and E. Isbister for Aircraft Flight Indicator andSystern is shown connected to the outputs of the above amplifiers. A balanced rectifier 33 is supplied with ap by amplifier 29 and an by amplifier 30. The varying direct current signal from 33 travels through leads 36 to operate a motor 31. The motor 3T actuates a conventional hydraulic servo system consisting of a sensitive valve 38, a power cylinder y35, and transmission cables 40 to control a vertical rudder surface 4|. Another balanced rectifier 34 is supplied with ai and an by amplifiers 3| and 30, respectively. The leads 35 from this rectifier 314 may be used to operate a similar servo system to turn the horizontal control surfaces of the craft.

If desired, instead of using a hydraulic servo system for operating the rudder 4| or elevator, as the case may be, the same may be operated by an electric servo system as shown in Fig. 10. In this system the balanced rectifier 34 is connected through the two outer leads 35 to the grids of grid controlled rectifier tubes |50 and |50'. A local alternating current supply feeds a voltage through transformer |52 to the plates of tubes |50 and |50' in phase opposition, while this supply acts through a network |53, central lead 35,V a divided resistor |54, and by way of the outer leads 35 to apply a phase shifted bias voltage to the grids of the tubes |50 and |50. This bias voltage is displaced approximately 180 with respect to the alternating current plate voltage.

When the output signal ai is of one phase corresponding to the location of the craft in the lower lobe I8 of Fig. 6, for example, then this signal will put a positive voltage on the grid of,

say, tube |50 at the same time that its plate swings positive. This causes tube |50 to conduct, thereby shorting the upper half of transformer |52, in effect, so that the lower half of this transformer is placed in parallel with a condenser |55. The presence of this condenser is reflected in the lower half of the secondary of transformer |52 as a large capacity having the effect of producing a substantially 90 leading current in the lower half of the secondary and in the connected winding |55 of a split phase induction motor |51, whose other winding |56, at this time, is supplied with current in phase with the supply I5 l. Thus the motor |51 operates in one direction to actuate control surface |58 in the proper manner to effect an upward movement of the craft toward the intersetcion of beams I8 and I9'.

If the craft were in beam I9', the lower tube would be caused to pass current thus effecting rotation of the motor |51 in the reverse direction as will be apparent. The greater the magnitude of the signal voltage, the greater the resultant motor speed will be, so that the motor speed and hence the rate of movement of the craft back to the desired glide path is substantially proportional to its deviation from this path. In order to prevent hunting of the craft about the desired glide path, an anti-hunt circuit may be used employing a generator |59 driven by motor |51 and supplying a Velocity voltage through a lead |60 to the input of power amplifier 3| in opposition to the signal Voltage ap.

The output of the demodulator 26 provides the servo voltage whose frequency is ai and whose phase is either 0 or 180 depending on which side of the true glide path the craft is located, and whose amplitude will vary with the rightleft position in space of the receiving equipment. The amplitude will increase with the angular deviation from the equi-signal path for signals up to a certain value, after which it will gradually reduce to Zero for continued deviation. The output of the demodulator 21 comprises Vthe reference Voltage of frequency an, of constant reference phase and substantially constant amplitude as provided by appropriate volume controls or by automatic volume control. The output of the demodulator 28 provides the servo voltage whose frequency is again ap, whose phase is either 0 or 180, and whose amplitude will vary with the up-down position in space of the receiver.

Consideration of the system as described above should make it appear that the demodulated output from demodulators 25 and 28 will have a Zero value, both as to amplitude and phase, when the aircraft is disposed along the equi-signal path, indicated in Fig. 3 for one coordinate by 0:0, When the receiving apparatus is at a position indicated in Fig. 3 by 0=01, the strength of signal from beam |8 is greater than that of beam i9. Consequently, there will be an alternating current output from demodulator 26 whose phase is 0 and whose amplitude, over a relatively broad angular range, will be roughly proportional to the angle 0. Similarly, when the receiving equipment is placed in the angular position 0:-0 of Fig. 3, the signal received from beam I9 will predominate over that of beam I8; the output of demodulator 26 will then comprise an alternating current of phase and of amplitude also roughly proportional to the angular deviation-0. Figs. 7, 8 and 9 indicate, by means of rotating vector diagrams, the three situations just described, wherein up indicates the output voltage of demodulator 26.

As above described, two pairs of beams will be needed in a complete instrument landing system; one pair for the localizer and one pair for the glide path. Generally the transmitting equipment for these two functions are located at the opposite ends of the runway, although they may be located at the same place. It will, therefore, generally be necessary to have two complete sets of transmitting apparatus, and their carrier frequencies may or may not be the same. Thus, two complete sets of receiving equipment on the craft may be necessary, one of which if tuned to the localizer transmitter and the other to the glide-path transmitter. To make one of the receivers required yfor a two separate receiver system, the channel consisting of lter 25, demodulator 28, and power amplifier 3l would be omitted from the receiver of Fig. 2.

Inasmuch as a two coordinate system requires a minimum of three signal channels, only two modulations, for example, z and sa, are necessary to separate these channels at the receiver. In this case one channel will be in the form wrap, so that filter 23 would pass ap, rejecting all other frequencies, and demodulator 26 would be eliminated. In general, if 11, is the number of channels employed, the minimum number of modulations necessary is (1i-1).

Fig. 5 shows a diagram of the variation of the direct current output of the balanced rectifiers 33 and 34, as a measure of the angular deviation of the plane from its proper course. When on course, there is zero current a negative angular deviation causes an increase in the plus sense, and a positive angular deviation an increase in the negative sense of the current. Over a reasonable range the change in current is roughly proportional to the angular deviation. Such direct current characteristics are admirably suited to cathode ray or meter indication or to the operation of electrical machinery, hydraulic machinery through electro-hydraulic interconnections, and to combination with other signals or take-off currents in the craft.

Since many changes could be made Vin the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l, In apparatus of the character described, a high frequency receiver adapted to receive initially modulated carrier signals that are further modulated with Xed phase reference and signal side bands, means for demodulating said carrier and separating said modulations in accordance with the frequency of saidpinitial modulations, means for demodulating said initial modulation and means for comparing said resulting reference and signal side bands as to phase and a servo mechanism controlled from said phase comparing means.

2. In an aircraft having a control surface, servo mechanism. for actuating said control surface, a receiver on said craft having means for detecting doubly modulated xed phase signal and reference voltages, means for comparing said separated voltages as to phase, filter means for separating said signal and reference voltages according to frequency, and motor means connected to said phase comparing means to be controlled from the latter, said motor means being connected in driving relation to said servo mechanism for controlling this mechanism and hence determining the operation of said control surface.

3. Instrument landing receiving apparatus comprising a high frequency receiver adapted for receiving a common carrier having a plurality of side bands, certain of said side bands containing a separation modulation and a signal modulation, and another of said side bands containing a different frequency separation modulation and a reference modulation, said receiver serving to detect the side bands, lter means for separating the respective side bands in accordance with the frequencies of said separation modulations, demodulators for demodulating the side bands to produce signal and reference voltages, said signal voltages being of fixed phase relationship with respect to each other, amplifying means for separately amplifying the said voltages, and balanced rectifier means for comparing the signal voltages as to phase with said reference voltage.

4. In the apparatus of the character described, a receiver adapted to receive carrier signals having a plurality of side bands, one of said side bands containing a sub-carrier modulation, together with a signal modulation and another of said side bands containing a sub-carrier modulation and a reference modulation, said receiver having means for detecting said side bands together with lter means for separating the latter in accordance with the frequency of said subcarrier modulations, means for demodulating said side bands to produce signal and reference voltages, balanced rectifier means for comparing versions of said signal and reference voltages, and a utility device operated in response to said balanced rectifier means.

5. In an aircraft position determining system, a receiver adapted to produce a plurality of output components of differing sub-carrier frequencies, said sub-carriers containing signal and reference modulations, filters for separating said sub-carriers according to frequency, means for demodulating said separated sub-carriers, means for comparing the signal and reference demodulated products as to phase, and a servo mechanism controlled from said last-named means for actuating the control surface of the aircraft.

6. In a craft radio position determining system for detecting the position of a craft in relation to directionally transmitted doubly modulated signals having signal components of opposite phase and a reference component of a phase Xed with respect to said signal components, radio receiving means for producing a plurality of output components of diierent sub-carrier frequencies with said signal components modulating a single subcarrier and said reference component modulating a second sub-carrier, a plurality of sub-carrier lters for selecting separate desired ones of said sub-carriers and means for demodulating said selected sub-carrier components and providing phase comparison between the resultant of said combined signal components and said reference component.

WILMER L. BARROW.

REFERENCES CITED The following references are of record in the .file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,004 Grieg Sept. 6, 1938 2,183,725 Seeley Dec. 19, 1939 2,253,958 Luck Aug. 26, 1941 2,256,487 Moseley et al Sept. 23, 1941 

